Silica coating composition for reflective sheeting

ABSTRACT

Retroreflective sheeting has been improved by a coating comprising silica and a transparent polymer selected from aliphatic polyurethanes, polyvinyl chloride copolymers having a minor amount of a comonomer containing at least one carboxylic acid or hydroxyl moiety, and acrylic polymers. Silica comprises 10-80 weight percent (10-70% in the case of polyacrylates) of the transparent protective coating. For example, a solution or dispersion of polyurethane is mixed with a silica sol and possibly other ingredients (e.g. wetting agent, UV light absorber and adhesion promoter), and the composition is coated onto the retroreflective sheet and dried. The coated retroreflective sheeting has superior soil and dew repellancy, retaining a higher percentage of original brightness after exposure to dirt and/or dew conditions.

This is a continuation of application Ser. No. 07/303,143, filed Jan.30, 1989, which is a div. of Ser. No. 151,541 filed Feb. 18, 1988, U.S.Pat. No. 4,844,976 which is a continuation-in-part of Ser. No. 023,375filed Mar. 9, 1987, U.S. Pat. No. 4,755,425.

TECHNICAL FIELD

This invention is in the field of retroreflective sheeting andspecialized coatings for such sheeting to impart soil repellency,wetting and other beneficial properties. Specifically, the inventionrelates to coatings containing silica and to processes for preparing thecoated compositions and for applying them to retroreflective sheeting.

BACKGROUND

There are several types of retroreflective sheeting: exposed lens (astaught in U.S. Pat. Nos. 2,326,634 and 2,354,018), embedded lens (seeU.S. Pat. No. 2,407,680; 4,367,920 and 4,505,967) and encapsulated lens(see U.S. Pat. No. 4,025,159). The drawings of the aforementionedpatents illustrate the various sheeting types and are incorporated byreference herein. Retroreflective sheeting, in general, comprises amultiplicity of lens elements and a reflector located behind the lenselements. The lens elements may be, for example, glass or ceramicmicrospheres in a polymeric base sheet. The reflector may be: a layer ofreflective metal (e.g. aluminum which has been vapor deposited on thelens elements or at some location behind the lens elements); flakes ofreflective metal embedded in a binder layer; or a dielectric reflectoras taught in U.S. Pat. No. 3,700,305.

Exposed lens retroreflective sheeting generally comprises a polymericbinder film in which is partially embedded a multiplicity of lenselements (e.g. glass microspheres), and a reflecting means disposed onthe back (embedded) side of the lens elements.

Embedded or enclosed lens sheeting may be comprised of: a backreflector; an overlying transparent spacing layer; a layer of small lenselements embedded in the spacing layer and in optical connection withthe reflector and spaced from it so as to place the reflector at theapproximate focal point of the lens elements; a transparent coveringlayer conforming to the front extremities of the lens elements andhaving a flat front face; and an optional transparent top layer forimproving weatherability.

Encapsulated lens retroreflective sheeting may, for example, becomprised of a plurality of glass microspheres having their backsurfaces partially embedded in a transparent polymeric layer with areflective layer adjacent to and disposed behind the embedded backsurfaces of the microspheres and another transparent polymeric layerover the front of the microspheres and bonded to the first transparentpolymeric layer in such a way as to result in air cells in front of themicrospheres.

Retroreflective sheeting has many applications. Exposed lens sheetingwith a dielectric reflector is used as a security film placed on top ofdocuments such as driver's licenses, passports, and automobile titles.They can bear identifying indicia such as a state seal or watermarkwhich can be seen at certain angles, but which do not interfere with thelegibility of the document. Exposed lens sheeting is also useful as asafety item on garments to make them reflective at night, such aspoliceman's uniforms, fireman's coats and jogging outfits.

Both enclosed and encapsulated lens sheeting are useful in applicationswhere they will be exposed to rain because the clear plastic layer whichlies over the microspheres or other lens elements helps to prevent waterfilms from adversely affecting the optics of the lenses. Thus, embeddedlens sheeting is used in license plates and validation stickers forautomobiles, and both encapsulated lens and enclosed lens sheeting havebeen used in highway signs and reflectors.

In use on highway signs and reflectors (e.g. pavement delineators placedon the road) it is desirable for sheeting to be soil resistant and dewrepellent. The accumulation of dirt and oil from the air and roadsurface onto retroreflective sheeting is detrimental to the retention ofits retroreflective brightness. Thus, the term "soil repellent" or "soilresistant" means reducing adherence of soil particles to the reflectorsurface.

It is well known that droplets of water (e.g., dew) can condense from ahumid atmosphere onto a cool surface. Such condensation formed upon thesurface of retroreflective sheeting impairs the transmission of lightthrough it. The term "dew modifier" as applied to retroreflectivesheeting means a material which is able to avoid a substantial decreaseof retroreflective brightness due to formation of water droplets on thesurface, for example by causing the dew or water to wet the surface andform a sheet or layer of water rather than droplets.

It is also desirable for the sheeting to be abrasion resistant,especially when it is to be used on the road surface. Retroreflectivesheeting is applied to road surfaces in the form of pavement markingtape and as the reflective element in pavement delineators or pavementmarkers. In these applications, the sheeting is actually contacted byvehicle tires as they traverse the pavement markers or lane markingtape. The force of the tires, combined with any road grit, can abradethe reflective sheeting.

The use of silica coatings to modify the surface of light transmissivematerials is known (e.g. antireflective coatings). However, the desiredcombination of soil repellency and dew modification have not yet beenobtained with a silica coating that is able to tenaciously adhere toretroreflective sheeting.

DISCLOSURE OF INVENTION

Retroreflective sheeting has now been improved by applying a polymercoating containing silica. The invention is summarized as a soilresistant and dew repellent reflective sheet having a transparentprotective coating comprising a mixture of silica and

a transparent polymer selected from the group consisting of:

aliphatic polyurethanes,

polyvinylchloride copolymers having a minor portion (i.e. less than 15weight percent) of a comonomer containing at least one carboxylic acidor hydroxyl moiety, and

acrylic polymers having a glass transition temperature (Tg) of -20° to60° C., preferably less than 45° C.,

wherein the silica comprises about 10-80 wt. % (10-70% in the case ofacrylic polymers) of the transparent protective coating.

As used above, the term transparent means transmitting at least 90% ofincident light in the visible spectrum (about 400-700nm wavelength).This has been determined using an IBM model 9420 UV-visiblespectrophotometer. The inventive protective coating was applied to thin(about 20-55 micrometers thick) polyester film. The dry protectivecoatings on the samples used had a thickness of about 2.5 to 7.6micrometers. The light transmittance through the uncoated polyester andthrough coated samples was measured at various wavelengths. In all casesthere was only a small difference in light transmittance between thecoated and uncoated polyester (≦4% ±) regardless of percent silica inthe protective coating or wavelength, indicating that transmittancethrough the protective coating was greater than 96%.

The term aliphatic polyurethane means a polyurethane derived from atleast one aliphatic polyisocyanate preferably without any aromaticpolyisocyanate. Aliphatic polyisocyanates known to the art are: 1,4cyclohexane bis(methyleneisocyanate); methylcyclohexylene diisocyanate;1,4-cyclohexanyl diisocyanate; dicyclohexylmethane diisocyanate;1,6-diisocyanato-2,2,4,4-tetramethylhexane;1,6-diisocyanato-2,4,4-trimethylhexane; ##STR1## wherein R₃ is --CH₃ or--C₂ H₅ ; and isophorone diisocyanate. The polyols used for aliphaticpolyurethanes may be polyester or polyether polyols.

The inventive protective coating can be made permanent. For purposes ofthis description, the term "permanent" means a coating which is notrubbed off easily by hand and which-is not readily removed by applyingcommon pressure-sensitive adhesive tape and lifting the tape off of thecoating.

The protective coating is the last coating applied to theretroreflective sheet. The retroreflective sheet itself may compriseseveral layers, as described previously, and the term "top layer" willbe used herein to refer to top-most layer of the retroreflective sheetjust before the protective coating is applied. Thus, as to theretroreflective sheet, the light must pass first through the protectivecoating and next through the top layer in order to pass through the lenselements and reach the reflector. Typical top layers for retroreflectivesheets are comprised of polyacrylate (e.g., polymethylmethacrylate),polyurethane (typically aliphatic polyurethane), polyvinyl (e.g.,polyvinyl-chloride) or polycarbonate polymers. The protective coating ofthis invention has been found to adhere very well to such top filmswithout affecting the retroreflective brightness of the sheet. Themodified sheeting has good soil repellency and anti-fogging properties(i.e., dew modification) and is suitable for use in pavement markings,roadway delineators, vehicle license plates and signing products.

The mixtures which are used as coating compositions for the protectivecoating can be formulated by mixing various concentrations of silica sol(aquasol, hydrosol or colloidal dispersion) with aliphatic polyurethaneor polyvinyl chloride copolymers in organic solvents or waterdispersion. The acrylic polymers are used in the form of aqueousemulsions. Silica sol colloid particle size is an important factor forpreparing a clear and transparent protective coating. The colloidparticles should be less than about 50 nanometers (nm) in diameter,preferably no greater than 20 nm in diameter. In the dry protectivecoatings, the silica particles agglomerate, but individual particles arediscernible and have been measured in the range of 0-40 nm diameter orparticle size, 15-34 nm to be precise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of brightness retention versus time in minutes forvarious retroreflective sheet materials exposed to dew conditions. Allof the retroreflective sheeting materials tested were of the enclosedlens type and had a vinyl top layer. All samples were exposed to thesame conditions in which water droplets condensed on the sheetingsurface, and the graph represents the initial loss in retroreflectivebrightness and the recovery of brightness over time. The solid line isfor the control, Scotchlite 680 reflective sheeting, and lines Nos. 1, 2and 3 represent various embodiments of the invention coated with amixture comprising 30% silica, with the remainder being comprised of apolymer and (in the case of lines 2 and 3) a wetting agent.

FIG. 2 is a graph of brightness retention vs. time, similar to FIG. 1,except that the polymer used was NeoCryl A601 acrylic polymer fromPolyvinyl Chemical Industries, Division of the ICI Group, and thereflective sheeting was Scotchlite 3870 reflective sheeting, fromMinnesota Mining and Manufacturing Company. The solid line representsthe uncoated control. Line A represents the same sheeting coated withNeoCryl A601 polymer. Lines B-D represent embodiments of this inventionusing coatings comprising NeoCryl A601 polymer and silica in varyingamounts.

FIG. 3 is a graph of brightness retention vs. number of test cycles forretroreflective sheeting exposed to soil resistance test conditions tobe explained hereinafter. All samples were exposed to the sameconditions of dirt and oil splashing, and the graph represents loss ofbrightness over time. The solid line is for the control, Scotchlite 3870reflective sheeting, and line A represents the same sheeting coated withNeoCryl A601 polymer. Lines B-D represent embodiments of the inventionusing NeoCryl A601 polymer and various amounts of silica in theprotective coating.

DETAILED DESCRIPTION

The raw materials selected for making the inventive protective coatingshould not adversely affect the retroreflective sheeting to which theyare applied. For example, they should not require drying conditionswhich would distort the sheeting and the relationship between the lenselements and the reflector. They should also not contain solvents orother ingredients which would dissolve or corrode the sheetingcomponents. Coating mixtures containing toluene, isopropanol and/orbutyl cellosolve have been found suitable for use with retroreflectivesheeting.

Colloidal silica dispersions are available commercially with differentcolloid sizes. Two such dispersions which have been found useful are:Nyacol 1440 colloidal silica, having a colloid size of about 14 nm and aconcentration of 40 weight percent silica, available from NyacolProducts, Inc., of Ashland, Md.; and Nalco 1140 silica, having acolloidal silica particle size of about 15 nm and a concentration of 40weight percent silica in water, available from Nalco Chemical Company ofOakbrook, Ill. Both are suspended in alkaline aqueous media. Othercolloidal silica dispersed in glycol ether solvent; e.g., Nalco 84SS-258silica from Nalco Chemical Co. having a silica particle size of about 20nm, can be used with polymer resins dissolved in organic solvent.

It is preferred that the polymer mixed with the silica sol is at leastsomewhat elastic, especially in cases where the retroreflective sheetingis to be applied to a flexible substrate, such as a rubber pavementmarker. Several of the polymers which have been found useful are givenbelow, (all monomer percents expressed being weight percent):

Aliphatic Polyurethanes:NeoRez R-960 Polyurethane, a flexiblepolyurethane sold in 33% by weight concentration in water by PolyvinylChemicals Inc., Division of ICI Group, Wilmington, Mass. Thispolyurethane is believed to be derived from the following monomers:dicyclohexylmethane diisocyanate ##STR2## Its molecular weight isbelieved to be at least 200,000. When the NeoRez polyurethane is used,it has also been found useful to include an aziridine crosslinkingcompound to improve the physical properties of the final coating. Thecross-linking compound used in the development of this invention hasbeen CX-100 cross-linker from Polyvinyl Chemicals Inc.

Permuthane U-6729 urethane resin sold in a 1:1 mixture of isopropanoland toluene at a concentration of 25% resin by ICI Specialty ChemicalDivision, a member of ICI Group. This resin is believed to be derivedfrom the following monomers: dicyclohexylmethane diisocyanate; adipicacid; neopentyl glycol; and 1,6-hexanediol. It is believed to have aweight average molecular weight (Mw) of about 33,000 and a numberaverage molecular weight (Mn) of about 11,000.

Permuthane U-23-327 urethane resin sold in a 1:1 mixture of isopropanoland toluene or methyl cellosolve at a concentration of about 35% resinby Beatrice.

Although the polyurethanes used in making this invention have not beenthe radiation curable variety, e.g. with acrylate or methacrylate endgroups, such polyurethanes may be used.

The molecular weight of the polyurethane is preferably over 20,000.

Polyvinyl chloride copolymers:

VROH copolymer, believed to have the following monomer content: vinylchloride (79-83%), vinyl acetate (5%), and vinyl alcohol (≦15%). It issaid to have 1.8-2.2% by weight hydroxyl content.

VMCH Copolymer believed to have the following monomer content: vinylchloride (85-88%), maleic acid (0.8-1.2%) and vinyl acetate (about 13%).Besides maleic acid, acrylic and methacrylic acids may be incorporatedinto the useful polyvinyl chloride copolymers.

VAGH partially hydrolyzed copolymer believed to have the followingmonomer content: vinyl chloride (89.5-91.5%), vinyl acetate (3%) andvinyl alcohol (5.2-6.5%). VROH, VMCH and VAGH copolymers are availablefrom Union Carbide Corporation.

Acrylic Polymers:

NeoCryl A601 polymer,

NeoCryl A612 polymer, and

NeoCryl A614 polymer.

These polymers are believed to be copolymers of methyl methacrylate andbutyl methacrylate having a weight ratio of methyl methacrylate units tobutyl methacrylate units in the polymer in the range of 0.25-0.6. Theratio for the three named polymers are believed to be as shown below:

    ______________________________________                                                  Weight ratio of monomeric                                                     units in polymer                                                              methyl methacrylate/butyl methacrylate                              ______________________________________                                        NeoCryl A601                                                                              0.33                                                              NeoCryl A612                                                                              0.43                                                              NeoCryl A614                                                                              0.47                                                              ______________________________________                                    

The molecular weights of the polymers is typically: Mw in the range of300,000-350,000 g/mole and Mn in the range of 50,000-70,000 g/mole.Analyses of two of the NeoCryl polymers indicates the followingmolecular weights:

    ______________________________________                                                       Mw    Mn                                                       ______________________________________                                        NeoCryl A612     323,000 60,000                                               NeoCryl A614     304,000 54,000                                               ______________________________________                                    

All three NeoCryl polymers were obtained from Polyvinyl ChemicalIndustries in water emulsion form. These emulsions have a pH similar tothat of the silica sols with which they are used, and they form arelatively stable mixture with the silica sols mentioned herein inexamples with the acrylates. These emulsions are characterized by: 32weight percent solids, pH of 7.7-8.0, viscosity at 25° C. of 100-200centipoise, and specific gravity of 1.0. They are also believed tocontain anionic surfactants and some organic solvents (11-14 weightpercent) said by the manufacturer to be coalescing solvents, as follows:

    ______________________________________                                                       A612     A614   A601                                           ______________________________________                                        aromatic naphtha 4.3%       3.7%   4.8%                                       2-butoxyethanol  1.3%       1.1%   --                                         2-(2-ethoxyethoxy)ethanol                                                                      2.2%       1.9%   --                                         2-methoxypropanol                                                                              5.0%       4.3%   --                                         ethylene glycol  1.0%       0.8%   --                                         diacetone alcohol                                                                              --         --     7.9%                                       ______________________________________                                    

Coalescing solvents have an affinity for the polymer and water and ahigher boiling point, at atmospheric pressure, than water.

The NeoCryl acrylic polymers can be blended with the NeoRezpolyurethanes to form a clear coating with colloidal silica. Such blends(e.g., 2:1 weight ratio of acrylate to polyurethane) have been found tohave excellent transparency (light transmittance), flexibility andadhesion.

Other compounds which have been found useful for incorporation into thecoating composition of this invention are:

Polyalkylene oxide modified dimethylpolysiloxanes which are polymericwetting agents sold under the trademark Silwet® by Union CarbideCorporation. The general formula for Silwet copolymers stated in UnionCarbide literature is: ##STR3##

Ultraviolet light stabilizers or absorbers, to improve the stability andweatherability of the protective coating. One suitable ultraviolet lightabsorber is Uvinul N-539 from GAF, New York, N.Y. whose literatureindicates that its formula is ##STR4## 2-ethylhexyl-2-cyano-3,3-diphenylacrylate Uvinul N-539 has been used with aliphatic polyurethanepolymers. Tinuvin 328 general purpose UV absorber by Ciba-GeigyCorporation of Ardsley, N.Y. has been used with acrylate polymers.Ciba-Geigy literature indicates that Tinuvin 328 light absorber has thefollowing formula: ##STR5## wherein R₁ and R₂ are t-amyl.

A retroreflective sheeting used in reducing the invention to practicewas a wide angle flat top sheet which comprises, in general: a backreflector of vapor deposited aluminum; an overlying transparent matrix;a light-returning layer of transparent microspheres embedded in thetransparent matrix and in optical connection with the reflector butspaced from it so as to place the reflector at the approximate focalpoint of the microspheres; and a transparent overlying solid coveringwhich conforms to the front extremities of the microspheres and has aflat front face. Such sheeting reflects a cone of light back toward alight source even though the incident light beam strikes the sheeting atan angle.

Wide angle flat top retroreflective sheeting may be made, for example,by a solution casting technique comprising the following process steps:(a) providing a paper carrier web coated with a release agent such aspolyethylene; (b) coating the release agent side of the carrier web witha 25% solids solution of fully reacted aliphatic elastomericpolyurethane of the polyester type in an isopropanol, toluene, xylenesolvent in sufficient amount to yield about a 50 micrometers dry filmthickness; (c) drying the coating from step (b) for example at about 90C for 15 minutes; (d) applying a bead bond coat about 5 micrometersthick of the same polyurethane material used in step (b) to the drycoating from step (c) and contacting the wet polyurethane surface withglass microspheres (e.g., about 20-120 micrometers diameter and 1.9-2.5refractive index); (e) drying the microsphere-coated web for example at93 C for 5 minutes; (f) coating a spacing layer polymer of the samealiphatic elastomeric polyurethane composition onto themicrosphere-covered web or sheet from step (e) in sufficient amount toyield a dry film thickness about equal to the focal length of themicrospheres; (g) drying the sheeting from step (f); (h) vapor coatingthe spacing layer with a specularly reflective material (e.g.,aluminum); (i) removing the paper carrier web; and (j) coating the backside of the reflective material with an acrylate-base pressure-sensitiveadhesive having a silicone coated release liner.

The soil resistant reflective sheeting of this invention is prepared bythe following steps:

A. The protective coating composition is prepared by mixing colloidalsilica with one of the selected polymers, such as polyurethane, and anyother desired ingredients, such as light stabilizer or cross-linkingagent;

B. A retroreflective sheet is provided, and it is coated with theprotective coating composition by conventional means, such as a wirewound bar, knife coating, coating roll or dip coating; and

C. The wet coating from step B is dried and cured by placing theretroreflective sheet in an oven.

The details of each of the above steps will now be explained. In onepreferred sequence for step A, the protective coating composition isprepared as follows:

(1) Dilute Permuthane U-6729 polyurethane solution with a 1:1 volumeratio mixture of toluene and isopropanol to about 25% concentration.

(2) Add light stabilizer in solution form to the solution from step (1).

(3) To the solution from step (2), slowly add colloidal silica (30%solids) and mix.

(4) To the mixture from step (3), add hydrophilic Silwet L-77 siloxanewetting agent and mix.

(5) Mix in minor amount (<10 weight percent) of Silane Xl-6106 adhesionpromoter from Dow Corning Corporation and mix gently to obtain ahomogeneous colloidal silica suspension.

It is generally desirable not to add the colloidal silica as the lastpart of step A, because small gel particles can be formed, and thecoating may become hazy. However, in the case of acrylate polymers, thesilica should be added last. The concentration of SiO₂ in the final drycoating is typically between about 15 and 60 weight percent.

When using the polyvinyl chloride transparent polymers, they arenormally furnished dissolved in organic solvent (e.g. 10 weight percentin methyl ethyl ketone). In that case, silica is also furnished in anorganic medium, e.g. Nalco 84SS-258 colloidal SiO₂ dispersion in glycolether.

The protective coating has good adhesion to retroreflective sheetingwith a polyurethane top layer. However, for retroreflective sheetingwith a polyacrylate top layer, it has been found desirable to pretreatthe sheeting with corona treatment to improve adhesion between thesheeting and the protective coating. A general discussion of coronatreatment of webs for improving adhesion is found in Cramm, R. H. andBibee, D. V., "The Theory and Practice of Corona Treatment for ImprovingAdhesion", TAPPI, Aug., 1982, pp. 75-78. The apparatus used in suchcorona treatment has been a Softal-Electric Company machine from WestGermany, Model 3025DW, having 1.2 Kw maximum power and electrode 0.56 mlong. Sheeting has been treated using a current of 180-300 milliamps anddrawing the sheeting through the apparatus at a rate of 10-100 m/min.for a sheet about 0.3 m wide. One typical treatment level is about 14Watts/ft² /min. (150w/m² /min).

The concentration ranges for the minor ingredients in the protectivecoating composition are: for the hydrophilic siloxane, 0-5 weightpercent, preferably about 3%; for the silane adhesion promoter 0-3%; andfor the light stabilizer 0-6 weight percent, preferably 2-4%.

The conditions for step C are generally about 50°-93° C. for 1-5 minutesin an air atmosphere. Generally, retroreflective sheeting having apolyurethane top layer can withstand somewhat higher temperatures, and acuring condition of 93° C. for 2-3 minutes is appropriate. However, inthe case of polyacrylate top layers, conditions are preferably about65°-71° C. for 1-3 minutes. Thickness of the final protective coating isgenerally less than 150 micrometers.

The invention will be further clarified by the following examples, whichare intended to be purely exemplary. In the examples, percentages ofprotective coating constituents are expressed as weight percent takingthe total of SiO₂ and the transparent polymer (e.g., polyurethane orpolyvinylchloride copolymer) as 100%.

Example I

In the case of the vinyl resins used in this invention, the stability ofthe protective coating composition (as measured by retention ofbrightness) has been found somewhat dependent on SiO₂ concentration.This is shown by the following table:

                  TABLE 1                                                         ______________________________________                                                                      Brightness                                      Sample  Vinyl Resin SiO.sub.2 Wt %                                                                          Retention (%)                                   ______________________________________                                        A       VAGH        20        98                                              B       VAGH        30        95                                              C       VAGH        40        86                                              D       VAGH        50        26                                              E       VMCH2       20        102                                             F       VMCH2       30        102                                             G       VMCH2       40        100                                             H       VMCH2       50        96                                              ______________________________________                                    

The above data show that the SiO₂ is quite stable in the VMCH₂ vinylresin from Union Carbide Corporation; whereas, above 30 weight percentconcentration, it becomes less stable in the VAGH resin. A whiteprecipitate was formed in some of the VAGH resin mixtures. Therefore,within the class of vinyl copolymers described, some experimentation isrequired to discern those which are useful and the concentration limitswithin which they are useful.

EXAMPLE II

In soil repellency tests, retroreflective sheeting samples weremechanically splashed with road dirt and water containing a trace amountof vehicle motor oil. The retroreflectivity retention of splashed anddried specimens was tested using a retroluminometer after each testcycle. The brightness retention corresponding to each test cycle isshown in Table 2 below:

                  TABLE 2                                                         ______________________________________                                        Sample    Control    23-A   27-A   27-B 27-F                                  ______________________________________                                        Wt % SiO.sub.2                                                                           0         33     33     25   60                                    Silwet    no         no     yes    yes  yes                                   Siloxane                                                                      Organic   --         --     x      x    x                                     base                                                                          H.sub.2 O base                                                                          --         x      --     --   --                                    % Initial                                                                     Brightness                                                                    Retained                                                                      after:                                                                        1st cycle 58         75     92     91   91                                    2nd cycle 42         68     88     86   87                                    3rd cycle 44         75     85     86   85                                    4th cycle 46         76     83     82   81                                    ______________________________________                                    

In the above table, the control was an encapsulated lens retroreflectivesheeting which is used in traffic signs, and all of the other sampleswere specimens of the same type of sheeting modified in accordance withthis invention. In each of the inventive samples, the transparentpolymer used was a polyurethane, and the term organic base means thatthe polyurethane was dissolved in organic solvent, while the term H₂ Obase means that the polyurethane was dispersed in water. The data showthat the inventive retroreflective sheeting samples retained more than80% of their initial brightness, while the uncoated control retainedonly about 46%. The data also indicate that the presence of a smallamount of the hydrophilic siloxane wetting agent may enhance the soilrepellency without adversely affecting brightness.

Example III

Dew tests were performed in accordance with the following procedure.Samples of retroreflective sheeting were laminated to aluminum panelswhich were placed in direct contact with the outside surface of arectangular, galvanized metal box filled with ice water. As the samplescooled, moisture in the air gradually condensed on their surfaces toform water droplets. A portable retroreflectometer was used to monitorthe brightness of the sheeting samples over time. The retroreflectometerused was made by Retro-Tech of La Mesa, Calif., and it reads in units ofcandela per lux-meters squared (cd/lux-m²). The results are indicated onFIG. 1 attached. The solid line is the plot for the control sample,Scotchlite 680 retroreflective sheeting. The other three plots are forspecimens which were the same as the control, except that they had beencoated with the inventive protective coating. In all three of thosecases, the concentration of SiO₂ was about 30%. For line no. 1, theprotective coating contained no siloxane wetting agent. For line 2, theprotective coating contained 3 weight percent Silwet siloxane wettingagent, and for the specimen represented in line 3, the protectivecoating contained 5 weight percent Silwet wetting agent.

From FIG. 1, one can see that the initial loss of brightness overapproximately the first five minutes was about the same for all four ofthe test specimens, but the recovery of brightness was quite different.It is evident that the wetting agent helps in the rapid recovery ofbrightness, since specimen 3 recovered about 87% of its brightness in 30minutes. In other similar tests in which all conditions were keptconstant except for silica concentration, it was found that specimenswith higher silica content appeared to retain more of their brightnessand recover a greater percentage of original brightness over time.

Thus, the protective coating has been made hydrophilic. That is, it isreadily wetted by water. Hydrophilic nature can be measured by thechange in contact angle of a drop of water on the surface, over time,i.e. the angle between the side of the drop and the surface at theperimeter of the drop. For example, a contact angle which increases overtime or remains high (e.g. over 70 ) would exemplify a hydrophobicsurface, whereas, a contact angle which decreases over time or remainslow would exemplify a hydrophilic material. That is, on hydrophilicsurfaces, water drops tend to spread out, while, on hydrophobicsurfaces, water tends to form beads.

Although the protective coating can be made hydrophilic, it resistsabsorption of water deep into the coating where it could cause opacity.In dew or rain conditions and even after immersion in water, theinventive retroreflective sheet remains clear and transparent, wet withwater drops on its surface.

EXAMPLE IV

Various embodiments of the retroreflective sheeting were made using thecoating compositions as shown in Table 3 below. In each case, thecoating composition was prepared by first mixing the polyurethanesolution and silica sol together, adding the light absorber, then addingthe siloxane wetting agent (if used). Finally, the silane adhesionpromoter (if used) was added. In each case the coating composition wasapplied to the same-type of retroreflective sheeting which was used inExample II. All of the coating compositions were coated by means of awire wound coating bar (wire 0.914 mm diameter). the coated samples weredried in a forced air oven at about 66° C. for about 3-4 minutes. Eachsample was subjected to an adhesion test similar to ASTM Test No.D-3359, using Scotch® 610 adhesive tape. The cross-hatch or latticepattern was made in the coating samples and the adhesive tape wasapplied and removed in a similar manner to the ASTM Test, but resultswere reported as adhesion failure percent. That is, by visualobservation, an estimate was made of how much of the protective coatingwas lifted off by the tape, indicating adhesive failure. The resultsreported are the average of more than three specimens. The adhesion testresults and the compositions of the samples are reported in Table 3below:

                                      TABLE 3                                     __________________________________________________________________________    Polyurethane            SiO.sub.2 sol (30% in                                                                    Tinuvin light                                                                        Siloxane                                                                             Silane* adhesion             25% Solids      Toluene/                                                                              glycol ether solvent)                                                                    absorber                                                                             wetting agent                                                                        promoter                              % of final                                                                           Isopropanol 1:1                                                                       % of final % of final                                                                           % of final                                                                           % of final                                                                             Adhesion            Sample                                                                             wt (g)                                                                            caoting solids                                                                       wt (g)  coating solids                                                                           coating solids                                                                       coating solids                                                                       coating                                                                                failure             __________________________________________________________________________                                                              %                   1    50  70     50      30         2                      0                   2    200 70     200     30         4                      0                   3    200 50     200     50         4                      0                   4    25  70     25      30         4      5               100                 5    25  70     25      30         4      3      2        10                  6    25  70     25      30         4      5      2        30                  7    25  70     25      30         4      8      2        100                 8    25  50     25      50         4      5               5                   9    25  50     25      50         4      3      2        0                   10   25  50     25      50         4      5      2        40                  11   25  50     25      50         4      8      2        100                 __________________________________________________________________________     *Added as 20% solution in isopropanol (75%) and H.sub.2 O (5%).               Weight reported is silane component only.                                

The above data show that, in general, increasing concentration of thesiloxane wetting agent is detrimental to adhesion, while adding thesilane adhesion promoter helps to overcome that effect. The adhesion ofthe inventive sheeting without either the wetting agent or the adhesionpromoter appeared to be excellent.

In another test, the same type of retroreflective sheeting was treatedwith corona discharge and then coated with colloidal silica sol anddried at 66° C. or 93° C. for 3 minutes. The adhesion test showed pooradhesion of the resulting silica coating to the top layer of thereflective sheet, and the silica could be removed completely by wipingit with wet paper.

EXAMPLE V

Accelerated weathering tests have been conducted in QUV and KRCweatherometers. The results of the first 500 hours testing showed90%-95% brightness retention for the inventive retroreflective sheet inwhich the protective coating contained 0.5%-1.5% Tinuvin 328 lightstabilizer.

EXAMPLE VI

Acrylic polymers have been tested by making several protective coatingcompositions with them, making the treated retroreflective sheeting ofthe invention and measuring the brightness of the sheeting both beforeand after the application of the protective coating composition. Thereflective sheeting used was Scotchlite 3870 high intensity sheeting. Inaddition, another acrylic polymer (Rhoplex E-1895) was used instead ofthe NeoCryl acrylic polymers in one of the experiments. The results areshown in Table 4 below.

                  TABLE 4                                                         ______________________________________                                        Colloidal Silica Containing Polyacrylate Coating                                             22-A 22-B    22-C   22-D 22-E                                  ______________________________________                                        NeoCryl A614     30 g   40 g    --   --   --                                  NeoCryl A612     --     --      30 g --   --                                  NeoCryl A601     --     --      --   30 g --                                  Rhoplex E-1895   --     --      --   --   20 g                                Nalco 1140 (40% SiO.sub.2)                                                                     24 g   13.7 g  24 g 24 g 25 g                                Coating Weight Percent SiO.sub.2                                                                50     30      50   50  --                                  Initial Brightness (cd/lux-m.sup.2)                                                            300    300     304  300  302                                 Brightness of Treated Sheeting                                                                 308    304     303  305   18                                 Brightness Retention (%)                                                                       100    100     100  100   6                                  ______________________________________                                    

The above data demonstrate that the inventive acrylate-silica coatingsdo not diminish the retroreflective brightness of the sheeting; whereas,the coating made with the Rhoplex polyacrylate emulsion resin wasunsatisfactory since it reduced brightness substantially.

EXAMPLE VII

Dew tests were performed like those described in Example III on samplesof the inventive reflective sheeting made with the acrylate polymers,and the results are shown in FIG. 2. The samples for lines B, C and Dhad 15 percent, 30 percent and 50 weight percent silica in theprotective coating respectively. FIG. 2 shows that the specimens treatedwith the mixture of acrylate polymer and colloidal silica lost lessreflectivity and recovered it more quickly than untreated sheeting. Withabout 50 weight percent silica in the coating, specimen D of thisExample lost virtually no brightness throughout the test. However,without the presence of the colloidal silica particles, the reflectivesheeting, even with the acrylate polymer (line A) lost substantialbrightness, and reflectivity recovery of the specimen was slow.

EXAMPLE VIII

Soil repellancy tests like those of Example II were also performed onsamples of the inventive reflective sheeting using the acrylate polymersin the protective coating. The data for these tests are shown in FIG. 3.As in the case of FIG. 2, the specimens for lines B, C and D utilized 15percent, 30 percent and 50 percent silica in the protective coatingrespectively. The figure shows that brightness retention of theinventive sheeting increases substantially as the proportion ofcolloidal silica is increased. Without the presence of the silica,specimen A, the soil resistance was poor, although somewhat better thanthe control.

Other embodiments of this invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. Various omissions, modifications and changesto the principles described herein, may be made by one skilled in theart without departing from the true scope and spirit of the inventionwhich is indicated by the following claims.

What is claimed is:
 1. A process for making a retroreflective sheethaving a protective silica coating comprising the steps of:A. preparinga protective coating composition by the steps of:1) providing atransparent polymeric composition selected from the group consisting ofacrylic polymers having a glass transition temperature of -20° to 60° inaqueous emulsion containing at least one solvent having a boiling pointhigher than that of water; 2) slowly mixing with the polymer from step(1) a silica sol, wherein the colloidal particle size is less than 50nanometers, in such a proportion to the polymeric composition from step(1) that the SiO₂ content is 10-70 weight percent, of the total of SiO₂plus the polymer solids from step (1); B. providing a retroreflectivesheet and coating said retroreflective sheet with the protective coatingcomposition from step A; and C. drying the coated retroreflective sheetfrom step B.
 2. The process of claim 1 which further comprises exposingthe retroreflective sheet of step B to corona discharge before it iscoated with the protective coating composition.
 3. A coating compositionwhich produces a transparent coating, comprising a mixture of colloidalsilica aquasol wherein colloid particle size is less than 50 nanometersand a polymer composition selected from the group consisting ofaliphatic polyurethanes having a molecular weight greater than 20,000 inwater dispersion, aliphatic polyurethanes having a molecular weightgreater than 20,000 in organic solvent, and acrylic polymers having aweight average molecular weight in the range of 300,000 to 350,000 and aglass transition temperature of -20° to 60° C. in aqueous emulsioncontaining at least one solvent having a boiling point higher than thatof water in which mixture the silica comprises: 30 to 80 weight percentof the total of silica plus polymer solids for aliphatic polyurethanepolymers; and 30 to 70 weight percent in the case of acrylic polymers.4. A coating composition comprising a mixture of colloidal silicaaquasol wherein colloid particle size is less than 50 nanometers and apolymer composition selected from the group consisting of acrylicpolymers having a weight average molecular weight in the range of300,000 to 350,000 and a glass transition temperature of -20° to 60° C.in aqueous emulsion containing at least one solvent having a boilingpoint higher than that of water in which mixture the silica comprises 30to 70 weight percent of the total of silica plus polymer solids.
 5. Acoating composition comprising a mixture of colloidal silica aquasolwherein colloid particle size is less than 50 nanometers and a polyvinylchloride copolymer having a minor amount, not more than 15 weightpercent, of a comonomer containing at least one carboxylic acid orhydroxyl moiety, in which mixture the silica comprises 10 to 80 weightpercent of the total of silica plus polymer solids.
 6. The coatingcomposition of claim 5 wherein silica comprises 30 to 80 weight percentof the total of silica plus polymer solids.